Casting, in which a liquid monomer is poured into a mold and transformed into a polymer, is a suitable method for the formation of transparent articles such as lenses, prisms and the like, because casting easily gives strainless isotropic molded articles. Extrusion molding can give fairly uniform and isotropic articles, though not so satisfactorily as with casting. In contrast to these, injection molding has heretofore been most unsuitable for obtaining homogeneous isotropic molded articles and, hence, in spite of being very efficient as a forming method, has not been used in molding those articles which are required to be homogeneous and isotropic. The reason is that in injection molding, a molten resin is injected at an extremely high speed into a mold, thereby producing a high shearing force which acts within the resin; accordingly, although not immediately apparent, there is always observed in injection molded transparent or translucent resin articles an intensive molecular orientation in the vicinity of the injecting gate, when the articles are examined between corssed polarizing plates. Such a molecular orientation often causes cracking of the molded articles. In addition, there is observed existence of flaws such as so-called jetting, flow marks, silver streaks and weld marks and frequently a sink mark. Consequently, efforts have heretofore been concentrated chiefly on the production of an article which reproduces more precisely the designed shape and on the reduction of time for each molding cycle, but hardly on the production of homogeneous and isotropic articles. A recent trend of the development in order to obtain less defective molded articles is along the line of complicating the construction and the control mechanism of injection molding machines, such as multi-stage control of the injection speed; detection and control of the extent of mold opening, injection pressure, mold clamping pressure and mold temperature.